ANDSCAPE L INES

INTRODUCTION

Tree coring is used to determine the age of trees and reveal information about structural health, growth rate, growth patterns, and

previous wounding or disease. Annual growth rings visible on a core

sample can be counted to determine tree age. The relative spacing of

the rings indicates growth rate and growth patterns.

A dendrochronologist is required to analyze tree cores in detail. The

analysis can reveal historic information about climate change. For

example, rings are spaced farther apart when sunlight and water are

abundant, when physical wounding occurs (such as fire damage), and

when competition changes occur (such as the death of an overstory

tree, which allows more sunlight to penetrate).

Some consider tree coring to be potentially harmful to trees. The

coring technique used to take a sample wounds the tree and may

introduce pathogens or open up existing pockets of infection within

the wood tissue that had been successfully compartmentalized by

the tree. Use of tree coring assumes that the tree is healthy enough

to seal off pathogens both chemically and physically before the

organisms have a chance to spread and possibly cause systemic

infection. APPLYING TREE CORING TO CULTURAL LANDSCAPE RESEARCH

Tree coring is a valuable investigative technique in cultural landscape

research when a mature tree appears to date from a known period

of significance, but historic documentation about the tree is lacking.

For example, in cultural landscapes where the period of significance is

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recent, questionable trees may have a caliper size of only six inches.

Tree Coring
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In addition, different growing conditions affect the

size of trees from an assumed average, making

age estimation difficult without coring. The

decision to core must consider physiological and

morphological characteristics of a particular

species, health status, potential vulnerability, such

as genetic susceptibility or local presence of

pathogens, management objectives, and the

proposed treatment for the landscape.

Tree coring was used as a research tool at Weir

Farm National Historic Site in Connecticut to

develop a restoration treatment plan for the site.

Historical research had not revealed whether

particular trees dated from the period of significance. The trees were cored and the data helped

planners decide whether to keep or remove the

trees in question.

PERFORMING THE TECHNIQUE

Equipment

When coring a tree, it is best to use the smallest

diameter bit possible for the increment borer.

(See Figure 1.) Borer bits range in diameter from

0.169 inches (4.3 mm) to 0.5 inches (12 mm). A

bit diameter of 0.169 inches is adequate for

determining age, but larger sizes are more

typically used for quantitative analysis in silviculture

research. The bit length used depends on the

radius of the tree with some extra length to

ensure the borer reaches the center of the trunk.

Bit lengths range from approximately 6 to 30

inches. The increment borer bit should be sharp

(a 3-thread bit penetrates more easily than a 2-

thread bit) and sterilized with rubbing alcohol.

SELECTING AN INCREMENT BORER

Figure 1. Photograph of an increment borer and extractor and

a diagram indicating how to select an appropriate increment

borer size. (Photograph courtesy of Forestry Suppliers, Inc.,

1996)

Three things to consider when ordering an increment borer

are length, diameter, and style.

Borer bit length depends on the size of the trees you will be

boring. Length is measured from the tip of the threads to the

end of the round section of the borer bit. This is the

maximum depth the bit will penetrate.

Core Diameter for the wood sample is determined by the

inside diameter of the opening at the threaded end of the bit.

.169 inches is commonly used for general forestry use, .200

inches for wood preserving testing, and .500 inches for large

amounts of wood for quantitative analysis.

2- or 3-Thread Style is a matter of personal preference. A 2-

thread borer has two threads on the cutting edge of the bit,

each originating 180o apart. A 3-thread borer has three

threads, each originating 120o apart. The 3-thread borer, due

to its higher pitch, will penetrate the wood deeper per

revolution than a 2-thread and also produce less friction

because more threads are pushing against the wood. It is

important to remember that the ease at which a borer

penetrates wood depends on wood hardness, friction

properties, and capability/strength of the user.

BORER BIT LENGTH

.169" (4.3 mm) .200" (5.15 mm) .500" (12 mm)
T R E E C O R I N G 3

Sterilization should be repeated between successive cores to prevent disease transmission between trees. After sterilization, the increment

borer is lubricated with a natural wax, such as

beeswax. If beeswax is coated along the borer to

the same length as the radius of the tree, the end

point of the wax can be used as a gauge of when

the centerpoint of the trunk is reached.

Drilling the Core

The best position for drilling is the one that

allows for the operator’s optimum leverage and

control. Tree coring is a strenuous activity, so the

preferred working position for the operator is at

stomach or chest height.

Figure 2. An increment borer is used to core a Douglas Fir

(Pseudotsuga menziesii) tree. San Juan Island National

Historical Park. (NPS, 1987)

Coring involves removing a core sample from

the trunk of a living tree using the increment

borer. (See Figure 2.) The core is a segment of

the cylindrical trunk, corresponding in length to

the trunk radius (the core extends from the

outer bark to the center of the trunk). To

obtain the core that is best for aging the tree,

drilling should occur at the lowest point on the

trunk before the transition to the root zone

(just above the root flair). This location contains

the greatest number of rings because it is the

oldest part of the tree. The higher up the tree

trunk, the fewer the number of growth rings. If

the core is taken too low in the root flair, the

core will be difficult to read. In this transition

zone, stem cells are modified as root cells and

the signature of rings becomes diffused. Approximately three to four feet above ground

level is ideal.

Once the drilling location is determined, the

increment borer is positioned so it can reach the

center of the trunk. After drilling, just beyond the

center (or up to the end of the beeswax), the drill

is reversed through one revolution to loosen the

core. An extractor is then inserted to remove the

core. The borer should be backed-out immediately so that proximate tissues will not swell. If

swelling occurs, removing the borer will be

difficult, if not impossible. The drill hole in the tree

trunk should be left untreated.

If the tree radius is larger than the bit length of the

increment borer, the center of the trunk cannot

be reached in a single core. The age of the tree

then must be interpolated. Age interpolation is

done by determining what percentage of the
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radius the borer can penetrate, calculating the

number of rings per inch in the extracted core,

then interpolating how many rings the remaining

uncored tissue will bear. The same interpolation

must be applied when a tree trunk is hollow and

a complete core cannot be extracted. It may be

very difficult or impossible to remove the increment borer from a hollow tree trunk.

Determining Age

A core should extend a little beyond the center

of the trunk to ensure that the centerpoint can

be visually identified. Wetting the core with water

or applying core dye may make the rings more

legible. Consecutive growth rings at the center

of the trunk (at the end of the core) appear as

increasingly acute single “parentheses,” which

become inverted beyond the centerpoint. The

midway point between the only “paired parentheses” marks the centerpoint; this is the reference point to either begin or end counting. Each

growth ring represents one year of life.

Some species are easier to read than others, due

to ring size, porosity of cells, presence of tannin,

or chemical discoloration. In the more difficult

cases it may be necessary to use staining treatments or dyes, magnifying lenses, and microscopes to visually enhance the rings for counting.

These procedures may demand the expertise of

a dendrochronologist.

REFERENCES

Forestry Suppliers, Inc. Haglof and Suunto Increment Borers. Jackson, MS: marketing catalog, to

order phone 1-800-647-5368.

Kramer, Paul Jackson. 1979. Physiology of

Woody Plants. New York: Academic Press.

Shigo, Alex, L. 1991. Modern Aboriculture:

A Systems Approach to the Care of Trees and

their Associates. Durham, NH: Shigo and Tree

Associates.

________. 1993. 100 Tree Myths. Durham,

NH: Shigo and Tree Associates.

________. 1994. Tree Anatomy. Durham, NH:

Shigo and Tree Associates.

The mission of the Department of the Interior is to protect

and provide access to our Nation’s natural and cultural

heritage and honor our trust responsibilities to tribes.

U.S. Department of the Interior

National Park Service

Cultural Resources

Park Historic Structures & Cultural Landscapes